1. Field of the Invention
The present invention relates to an aluminum alloy die casting method which allows for the manufacture of a weldable casting.
2. Description of the Related Art
Casting a molten aluminum alloy in a die in atmospheric air using only gravity as a force is called a xe2x80x9cgravity die casting methodxe2x80x9d or simply a xe2x80x9cdie casting methodxe2x80x9d. Such a casting method has been effectively used to manufacture aluminum alloy castings for use as parts of a two-wheeled or lightweight vehicle body, or for engine parts.
However, the gravity die casting has problems, as in a sand mold casting method, such that its long cycle of casting limits productivity, the produced castings have poor dimensional accuracy, and a post heat-treatment is required to improve the strength of the castings.
In an attempt to solve these problems, it became necessary to consider adopting a die casting method which provides improved dimensional accuracy and has an extremely short cycle of casting. The principle of this die casting method resides in press-injecting molten metal into a cavity of a die at a high speed and pressure. In this die casting method, since molten metal is injected at a high speed, air is entrapped in the molten metal and remains as bubbles in a casting, with the result that the bubbles produce blisters on the surface of the casting upon heating of the latter. On the other hand, this die casting method is advantageous in that since molten metal is injected at a high pressure, a die-cast product obtained by the method has a dense structure and a flat cast surface, which lead to increased strength of the product and thus eliminate the necessity for giving a post heat-treatment to the product.
For manufacturing a three-dimensional structure such as a two-wheeled vehicle body, it becomes necessary to weld castings together. A casting obtained by the above-described gravity die casting is weldable but not a casting obtained by the latter die casting method.
Various improved die casting methods have been proposed for the manufacture of weldable die-cast products. An example is Japanese Patent Laid-Open Publication No. HEI-4-172166 entitled xe2x80x9cMETHOD OF MANUFACTURING ALUMINUM CAST PARTS FOR BRAZINGxe2x80x9d. According to this method, as shown in the drawing figures of the publication, a flow rate of molten metal at a gate (gate velocity) is switched stepwise between a low flow rate ranging from 0.3 m/sec to 0.6 m/sec for a first half of processing and a high flow rate ranging from 10 m/sec to 30 m/sec for a latter half of the processing.
However, in an associated die casting machine, an expensive control mechanism and a highly advanced control technique are required to switch the moving speed of a piston in the course of forward movement thereof. Further, the die casting machine is required to have increased rigidity so that it can withstand a large accelerating or decelerating force generated due to the change in moving speed of the piston. There has also been known a die casting machine having two cylinders that can be selectively used for effecting the change in moving speed of a piston. Although such a die casting machine facilitates the control of the movement speed of the piston to some extent, it becomes large in size.
It is therefore an object of the present invention to provide an aluminum alloy die casting method which enables the manufacture of a weldable die-cast product without requiring expensive modifications to an existing die casting machine and a highly advanced control technique.
To achieve the above object, according to the present invention, there is provided an aluminum alloy die casting method comprising the steps of providing a die casting machine having a gate for allowing passage of molten aluminum alloy, setting a flow rate of the molten aluminum alloy at the gate to be in a range of 5 m/sec to 15 m/sec, and press-injecting the molten aluminum alloy into a cavity of a die.
With this arrangement, it becomes possible to obtain a weldable casting with no entrapment of air. For example, an aluminum alloy for a vehicular part, which is formed of a die cast product manufactured by the die casting method of the present invention, is weldable and also dense in structure. As a result, these vehicular parts formed of the die-cast products are manufactured on a large scale at a low cost.
As will be described in detail with reference to FIG. 2, the amount of gas entrapped in a casting gradually increases with the increase in the flow rate of molten metal passing through the gate. It significantly increases particularly when the flow rate exceeds 15 m/sec. Also, as will be described in detail with reference to FIG. 3, the yield strength of the casting is maximized when the flow rate of molten metal passing through the gate is in a range of 7 m/sec to 9 m/sec. The yield strength becomes small when the flow rate is reduced to 5 m/sec, although the amount of gas entrapped in the casting is small at such a flow rate, 5 m/sec. This is believably because molten metal gets cooled and loses some of its fluidity during filling into a mold cavity at a flow rate less than 5 m/sec, thereby causing incomplete filling. In other words, at a flow rate less than 5 m/sec, the die casting cannot fully perform its operation of molten metal filling at a high speed and pressure. In aluminum alloy die casting, it is thus necessary to keep the flow rate at a pouring gate in a range of 5 m/sec to 15 m/sec.
It may be readily appreciated by a skilled artisan that the shape and size of a casting reflects upon the die casting. For example, a time required to fill up a mold cavity becomes larger with the increase in size of a casting. When producing a large-sized casting, there may occur an inconvenience such that an initially injected part of molten metal is solidified before the cavity is completely filled up, or a portion having a thin thickness is solidified in a far shorter time compared to other portions of the casting.
Although the inventive molten metal flow rate at the pouring gate is determined irrespective of the shape and size of a casting, it would be more practical if the shape and size of the casting are taken into consideration.
To this end, by using F. C. Bennett""s equation which is based on the thought that filling should be completed within a time of 70% of the time required for complete solidification of molten metal, the present inventors have decided to establish a simplified equation for determining a sectional area of the gate with the flow rate of molten metal passing through the gate, the specific heat of molten metal, the temperature of molten metal, the thickness of a casting, and the like taken into consideration.
The following equation (1) is given by multiplying F. C. Bennett""s equation by a modification factor a. A relationship of t=0.808T2 is obtained by substituting numerical values in variables of the equation, for example, 0.23 and 650 into xe2x80x9ccxe2x80x9d and xe2x80x9cTmxe2x80x9d, respectively.
The density (2.35 g/cm3) of molten metal becomes smaller than the density (2.7 g/cm3) of the metal in solid state at room temperature due to thermal expansion of the metal.                     t        =                  α          ·                                                    c                ⁡                                  (                                      Tm                    -                    Ts                                    )                                            +              Ga                                      4              ⁢                              λ                ⁡                                  (                                      Tm                    -                    Td                                    )                                                              ·          ρ          ·                      T            2                                              (        1        )            
where
t (filling time): (sec)
xcex1 (modification factor): 1.5
c (specific heat of molten metal): 0.23 (cal/gxc2x7xc2x0 C.)
Tm (temperature of molten metal): 650 (xc2x0 C.)
Ts (temperature of solid line): 598 (xc2x0 C.)
Td (surface temperature of die): 300 (xc2x0 C.)
Ga (latent heat of molten metal): 94 (cal/g)
xcfx81 (density of molten metal): 2.35 (g/cm3)
xcex (heat conductivity): 0.33 (cal/cmxc2x7sxc2x7xc2x0 C.)
T (thickness of casting): (cm)
The following equation (2) (W=xcex3xc2x7100vnxc2x7txc2x7S) is obtained for a casting having a weight W which is manufactured by filling a cavity with molten metal through a gate having a sectional area S at a flow rate v1 or v2 of the molten metal passing through the gate for a filling time t. Numeral 100 on the right side of the equation is a value for converting the unit xe2x80x9cmxe2x80x9d (meter) into the unit xe2x80x9ccmxe2x80x9d (centimeter).
W=xcex3xc2x7100vnxc2x7txc2x7Sxe2x80x83xe2x80x83(2)
where
W (weight of casting): (g)
xcex3 (specific gravity of casting): 2.7 (g/cm3)
vn:
v1 (flow rate at gate): 5 (m/sec)
v2 (flow rate at gate): 15 (m/sec)
t (filling time): 0.808T2 (sec)
From the above equation (2), the sectional area S is given by the following equation (3). In addition, the equation (4) below is given by substituting numerical values in variables of the equation (3), for example, 2.7, 5, and 15 in xcex3, v1 and v2, respectively.                               W                                    γ              ·              100                        ⁢                          v2              ·              0.808                        ⁢                          T              2                                      ≤        S        ≤                  W                                    γ              ·              100                        ⁢                          v1              ·              0.808                        ⁢                          T              2                                                          (        3        )                                          W                      3272.4            xc3x97                          T              2                                      ≤        S        ≤                  W                      1090.8            xc3x97                          T              2                                                          (        4        )            
It is thus preferred that the present invention further includes the step of setting the sectional area S (cm2) of the gate to be in a range obtained by the above equation (4), where W (g) is the weight of a casting and T (cm) is a typical thickness of the casting.
The simplified equation (4) is provided to determine the sectional area of the gate with the flow rate of molten metal passing through the gate, the specific heat of molten metal, the temperature of molten metal, the thickness of a casting, and the like taken into consideration. In the equation (4), the term on the left side is the sectional area of the gate obtained when the flow rate of molten metal passing through the gate is 5 m/sec, whilst the term on the right side is the sectional area of the gate obtained when the flow rate of molten metal passing through the gate is 15 m/sec. By virtue of the equation (4), the sectional area of the gate can be simply determined without complicated calculation, thereby making it possible to reduce the number of design steps for machine installation and adjustment steps during test run.
Desirably, the method according to the present invention further includes the step of coating the die with a mold releasing agent which contains an inorganic component and graphite as main components and a volatile component of less than 30 wt % as a sub-component but does not contain moisture.
Use of a mold releasing agent is effective to smoothly release a casting from a die. The mold releasing agent is required to contain a volatile component such as a high polymer synthetic oil. The volatile component makes the mold releasing agent, which has high stickiness, to remain stuck on the mold. The volatile component is, however, thermally decomposed to produce a hydrogen gas. When the content of the volatile component is more than 30 wt %, a large amount of gas is produced to exert an adverse effect on weldability of the casting. When it is less than 15 wt %, the stickiness of the mold releasing agent is insufficient. Accordingly, in the inventive aluminum alloy die casting method, the content of the volatile component, which volatilizes at a temperature of 700xc2x0 C. or more, should fall in a range of 30 wt % or less, preferably in a range of 15 to 30 wt %.
The die casting method according to the present invention may further include the step of arranging the gate to be positioned at that portion of the casting which is used for weld connection to other structural parts. A casting defect is less induced at the gate portion of the casting, as compared with other portions of the casting. Accordingly, a desirable welded structure with less welding defect is obtained by welding at the gate portion of the casting to other structural parts.